Two-component polyurethane forming systems are well known. Such systems typically comprise as a first component an NCO-terminated prepolymer and as a second component a polyfunctional alcohol, i.e., polyol. Typically, these components are reacted in the presence of a catalyst, the hydroxyl groups of the polyol reacting with the NCO groups of the NCO-terminated prepolymer to form a polyurethane. Alternatively, it is possible to form polyurethane compositions in the absence of a catalyst. See, e.g., U.S. Pat. No. 4,170,559, which is incorporated herein by reference.
One useful application for the polyurethane compositions formed in the manner described above is as a sealing resin or potting resin employed in the manufacture of filtration and separation equipment. In particular, the polyurethane compositions are useful as sealing resins in the manufacture of separatory devices used in industrial filtration applications and biomedical applications as well as in certain food, drug and cosmetic applications.
However, in such applications, the choice of sealing or potting resins is severely limited by the fact that the resins can not be toxic during use. The residual presence of toxic catalysts, for example, is to be avoided. The presence of such catalysts may create the risk of catalyst exudation, resulting in undesirable contamination of the fluids passing through the filtration or separation equipment.
It has become desirable to prepare formed polyurethane compositions for use, e.g., in the above applications, employing prepackaged polyol/catalyst solutions. This procedure is economically advantageous, allowing use of a complete polyurethane system in two packages (isocyanate terminated prepolymer and polyol/catalyst solution) instead of three (polyol, isocyanate terminated prepolymer and catalyst). It is therefore desirable to prepare polyol/catalyst mixtures where the catalysts are soluble in various polyols and which remain in solution even when the polyol is exposed to freezing temperatures. It is also desirable to prepare polyol/catalyst mixtures wherein catalytic efficiency is maintained over extended periods of time.
Recently, in connection with certain applications, it has also become desirable to employ two-component polyurethane forming systems wherein the NCO-terminated prepolymer is derived from aliphatic isocyanates rather than aromatic isocyanates. It has been suggested that employing aliphatic isocyanates avoids the possibility of toxic aromatic amines being formed by hydrolysis of aromatic isocyanates.
Therefore, polyurethane forming compositions and formed polyurethane compositions which can employ NCO-terminated prepolymers derived from aliphatic isocyanates have become increasingly desirable.
A number of catalysts are known to increase the reaction rate between the hydroxyl groups of the polyol and the NCO groups of the NCO-terminated prepolymer, e.g., catalysts such as aliphatic and cycloaliphatic tertiary amines, certain soluble metal compounds and certain acids.
Polyurethane forming compositions and formed polyurethane compositions employing known catalysts have certain significant drawbacks. For example, while composition formation reactions employing the aliphatic and cycloaliphatic tertiary amine catalysts discussed above are known to exhibit increased hydroxylisocyanate reaction rates, compositions employing such catalysts are unsuitable for use in the filtration and separation equipment under consideration here because of their cytotoxicity.
Polyurethane forming systems and formed polyurethane compositions employing amidine-metal complexes and amine-metal combinations are also known. See, e.g., U.S. Pat. Nos. 4,006,124, 4,115,320 and 4,150,212. However, compositions employing such catalysts are also unsuitable for use in the filtration and separation equipment under consideration here because of their cytotoxicity.
The hydroxyl-isocyanate reaction rate is also known to be slightly increased in polyurethane formation reactions employing strong acids, as illustrated by J. Saunders and K. Frisch, Polyurethanes, Chemistry and Technology at 211-215 (1962). However, acids in general are very cytotoxic when introduced into the blood stream and their residual presence in, e.g., potting resins, adhesives, coatings, sealants or encapsulants used in the filtration and separation equipment under consideration here is to be avoided because of the risk that they will exude and contaminate the fluids passing through the equipment.
Polyurethane forming compositions and formed polyurethane compositions employing metal compounds such as tin octoate or ferric acetyl acetonate have also been known to increase the hydroxyl-isocyanate reaction rate in polyurethane formation reactions. While polyurethane compositions employing tin octoate have been found to be non-toxic, it has been found that tin octoate is hydrolytically unstable and must be added to the polyol on site rather than during packaging of the polyol. Ferric acetyl acetonate is toxic at levels of about 0.1% by weight and higher and imparts a dark red color to the polyurethane.
Other polyurethane catalysts, i.e., stannous carboxylates, ferric acetylacetonate, titanium alcoholate, etc., are very efficient catalysts but also very hydrolytically unstable. When dissolved in polyols their solutions must be used soon after their preparation or they will lose most or all of their catalytic activity through oxidation and hydrolysis by the water generally present in the polyols. For a polyol solution of the catalyst that remains stable and does not change in activity (i.e., gel-time, non-flow time and demold time) the catalyst has to be oxidation resistant and hydrolytically stable. Its activity has to remain constant over long periods of time at room temperature and higher. Conditions which are generally encountered when such solutions are stored for a long period of time in a warehouse.
For this reason, it is desirable to have catalyst-containing polyurethane forming compositions which are hydrolytically stable in solutions of different polyols at different water contents.
Polyurethane forming compositions and formed polyurethane compositions employing N-N-N'-N' tetrakis (2-hydroxypropyl) ethylene diamine, generally known as QUADROL (a trademark of Wyandotte Chemical Co.) are known. See, e.g., U.S. Pat. No. 4,224,164. However, several drawbacks are associated with such systems. Large amounts of QUADROL catalyst are generally required to be used in the preparation of commercial polyurethane. For example, in systems employing prepolymers derived from aromatic isocyanates, amounts up to about 15% by weight, based on the weight of the polyol, are required. Employing such large amounts of catalyst is undesirable, often requiring frequent reformulation of the polyurethane compositions to maintain consistent physical properties. Further, since the QUADROL catalyst is tetrafunctional, a high degree of crosslinking is introduced which also may significantly change the physical properties of the polyurethane compositions. Finally, use of the QUADROL catalyst, which possesses a low molecular weight and high hydroxyl value, requires the use of large amounts of the isocyanate-terminated prepolymer, which is expensive and therefore economically disadvantageous.
Polyurethane forming compositions and formed polyurethane compositions employing ricinoleic acid as a catalyst are known to be non-toxic and are hydrolytically stable. However, large amounts of ricinoleic acid catalyst, e.g., amounts up to about 30% by weight, based on the weight of polyol, are generally required to be used in the preparation of commercial polyurethanes. See, e.g., Kroplinski et al, U.S. patent application Ser. No. 90,206, filed Nov. 1, 1979, now U.S. Pat. No. 4,256,617.
The search has therefore continued for polyurethane compositions which are derived from aliphatic or aromatic isocyanates, which are non-cytotoxic and which can be prepared employing relatively non-cytotoxic catalyst than has heretofore been possible and for stable polyol/catalyst solutions useful in preparing such compositions. The present invention is a result of that search.